Lineage analysis serves to define progenitor-progeny relationships in embryonic development. These analyses inform our understanding of the types and timing of choices that progenitors make towards generating the complexity of cell fates observed in the adult organism. In the embryonic CNS, lineage analysis has provided a fundamental substrate on which scores of models are built upon, and have provided an understanding of how the embryonic neural tube progenitors, through a series of molecularly guided decisions, generate the enormous complexity of cell types observed in the adult CNS. All methods of lineage tracing rely on a single fundamental principle - that of being able to indelibly label a progenitor, or subset of progenitors. Most recently, this has been accomplished through the use of site- specific recombinases. Site-specific recombinases can be driven by gene-specific promoters in discrete regions of the developing neural tube. In these cells, the recombinase activates a reporter in a permanent manner, thereby permitting tracking of those cells, and their descendent lineages, through subsequent developmental stages. Recombinase based lineage analysis has transformed our understanding of embryonic CNS development. Recombinase based lineage analysis is subject to one critical limitation. The specificity of the method is wholly reliant on the specificity of the promoter/regulatory elements used to drive the recombinase. Towards improving the specificity of this method, we and others have developed intersectional and inducible methods to label more selective groups of progenitors. These improvements have led to even more refined fate maps in various regions of the embryonic CNS. Despite these improvements, one caveat remains - genes and their cognate regulatory elements are often expressed in progenitors, as well as in related or unrelated postmitotic neuron populations. Expression of recombinase drivers in postmitotic neurons obfuscates lineage analysis, as recombination occurs in the postmitotic cell regardless of which progenitor it was derived from. Thus, in such cases, progenitor-progeny relationships cannot be accurately ascertained. Here we propose to develop a new platform for lineage analysis, termed Progenitor anchored intersectional fate mapping (PRISM) which circumvents the aforementioned limitations. In Specific Aim 1, we will establish the validity of the PRISM approach. In Specific Aim 2, we will use this approach in proof-of- principle experiments to resolve the lineage of the key molecule Shh in the ventral forebrain wherein Shh is expressed contemporaneously in progenitor cells as well as in postmitotic neurons. The proposed strains add to the growing conditional toolbox for lineage analysis. Since most recombinase drivers are subject to limitations, this method will be broadly applicable for studying important lineage related questions.